1. Field of the Invention
The present invention relates to a video encoder and video encoding method and more specifically, is preferably applied to a video signal encoder, for instance, used for a digital broadcasting system.
2. Description of the Related Art
Various kinds of compress-coding methods have been proposed for decreasing video and audio information. A method called moving picture experts group phase 2 (MPEG2) has been introduced as a representative one. A digital broadcasting system, which compress-encodes video and audio broadcasting data with employing the MPEG2 method and broadcasts the resultant via a ground wave and satellite wave, has been started.
FIG. 1 generally shows a video signal encoder 50. A video signal D1 is inputted from a video signal supply device (not shown) such as a video tape recorder or the like, to a prefilter 51. The video signal D1 is subjected to a band restriction process corresponding to a frequency characteristic control signal D55 supplied from a quantization rate control section 55 in the prefilter 51. Thereby, the high frequency component of the video signal is reduced and the video signal is outputted to a picture element number converting section 52 as a band restricted video signal D51. Note that, the high frequency component of the video signal represents relatively minute parts of an image. The high frequency component is reduced to the degree that the minute section of the image is omitted, and the bandwidth of the video signal is reduced, while only a small influence is applied to the entire image.
The picture element number converting section 52 executes a picture element number converting process on the inputted band restricted video signal D51. Assuming that the number of horizontal picture elements of the video signal D1 inputted to the encoder 50 is the number M of horizontal picture elements, the picture element number converting section 52 reduces the number of horizontal picture elements of the band restricted video signal D51 obtained by restricting the band of the video signal D1 to the number N of reduced horizontal picture elements with a relation of N&lt;M. The resultant signal is transmitted to a encoding section 53 as a picture element number converted video signal D52. The number N of the reduced horizontal picture elements is set to a large value in a program which requires a high picture quality: and to a small value in a program which does not require a high picture quality, based on the contents of the program in the video signal D1.
The encoding section 53 executes on the picture element number converted video signal D52, a movement compensating process, a discrete cosine transform (DCT) process, a quantization process and a variable length coding (VLC) process, in order to transmit the resultant to a buffer 54 as variable length coded data D53. At this time, the encoding section 53 adjusts a quantization rate in the quantization process based on a quantization control signal D56 supplied from a quantization rate control section 55. Further, encoding section control information D57 is supplied from a encoding section control circuit 57 to the encoding section 53. The encoding section 53 sets a coding timing and a movement vector search range in the movement compensating process according to the encoding section control information D57.
When the movement of the video image in the video signal D1 is large, or the video image is complicated, the generated code of the variable length coded data D53 obtained by compress-coding the video signal D1 is increased. The video signal having the large number of generated codes by such a compress-coding is referred to as a video signal with a high degree of difficulty in compression. The degree of difficulty in compression varies based on the contents or scenes of the program in the video signal. Therefore, the quantization value of the quantization process in the encoding section 53 and the band restriction in the prefilter 51 are controlled based on the variable length coded data D53 occupied in the buffer 54. Thus, the number of generated codes for predetermined term, for example, each group of pictures (GOP) is controlled so as to be kept constant.
In other words, the quantization rate control section 55 constantly monitors the accumulated state of the variable length coded data D53 in the buffer 54 in order to obtain an accumulated state as occupation rate information D54. Then, the quantization rate control section 55 generates the quantization control signal D56 and the frequency characteristic control signal D55 based on the occupation rate information D54 in order to supply them respectively to the encoding section 53 and the prefilter 51. Thereby, the section 55 controls the number of generated codes of the variable length coded data D53 constant for each predetermined period.
In the above video signal encoder 50, the bandwidth reduced by the prefilter 51 varies according to the degree of difficulty in compressing the video signal D1. On the other hand, the number N of horizontal picture elements reduced by the picture element number converting section 52 is constant according to the contents of the program in the video signal D1. Therefore, the number N of horizontal picture elements of the picture element number reduced video signal D52 happens to exceed the number of horizontal picture elements necessary for representing the video image of the band restricted video signal D51. In this case, the number of picture elements exceeding the requirement for representing the video image of the band restricted video signal D51 are to be encoded. Accordingly, the quantization value in the quantization process is unnecessarily increased and disadvantageously results in the deterioration of the picture quality.